322 THE AMERICAN MONTHLY [Dee 
Navicula crassinervis. Striae 34 to 35 in 0:01 mm, 
according to Van Heurck. This has proved a most delic- 
ate object with the 4mm., both dry andin realgar. With 
N. A. 1:3 and 1:4 realgar mounted valves are sharply 
resolved into dots, but the transverse striae have alone 
been seen with the dry lens. 
Hyalodiscus subtilis. In a mixture of monobromide of 
naphthaline and balsam. Dotted structure on outer zone 
well seen, although faint and difficult near the edge of the 
disc. In balsam mounts the structure appears still fainter, 
but nevertheless may be traced nearly to the outer edge, 
where it runs at about 76,000 to the inch. 
Surirella gemma Ehrbg. In realgar the beading has 
been seen beautifully defined with the valve arranged 
longitudinally on the sharply focussed edge of the lamp 
flame. Specimens mounted dry, in balsam, and in quini, 
dine, have been also examined, but their complete resolu- 
tion has proved a much more difficult matter. 
Colletonema vulgare. Moller’s balsam type slide. This 
has been most carefully studied with the 4 mm. The 
resolution is very faint, and requires particularly exact 
focal adjustment, but when once seen it can be held fairly 
steadily without any great difficulty. Dr. Van Heurck 
writes of this diatom, “Stries fines, delicates, les moy- 
ennes faiblement radiantes, les terminales paralleles- 
environ 34 en 1 c. d.m.; les stries medianes plus fortes, 
plus ecartees, 24.en 1c. d. m. et plus radiantes,”’ 
Navicula major. Moller’s balsam typeslide. The full 
resolution of the structure of the bands on the hoop of 
this diatom is by no means easy, even with the Zeiss 3 
mm. apochromat of N. A. 1:4. Notwithstanding this, 
the resolution is carried very far by the 4 mm., thestriae 
appearing remarkably black, crisply defined, and well 
separated, their beaded nature being quite recognizable, 
although not so fully revealed as with the oil-immersion. 
On this specimen the striae alone are just visibly separat- 
